Method and apparatus for wafer cleaning

ABSTRACT

An apparatus for wet processing individual wafers comprising; a means for holding the wafer; a means for providing acoustic energy to a non-device side of the wafer; and a means for flowing a fluid onto a device side of the wafer.

RELATED APPLICATIONS

The present divisional application is related to, incorporates byreference and hereby claims the priority benefit of the following U.S.Patent Applications, assigned to the assignee of the presentapplications: U.S. patent application Ser. No. 09/891,849, filed Jun.25, 2001 which is continuation-in-part of U.S. patent application Ser.No. 09/603,792, filed Jun. 26, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of cleaning of a substratesurface and more particularly to the area of chemical and megasoniccleaning of a semiconductor wafer.

2. Discussion of Related Art

In semiconductor wafer substrate (wafer) cleaning, particle removal isessential. Particles can be removed by chemical means or by mechanicalmeans. In current state of the art, particles are usually removed byboth a combination of mechanical means and chemical means. The currentstate of the art is a batch process that places a number of wafers intoa bath filled with a liquid and to apply high frequency (megasonic)irradiation to the liquid. Megasonic cleaning uses a ceramicpiezoelectric crystal excited by a high-frequency AC voltage that causesthe crystal to vibrate. The vibration causes sonic waves to travelthrough the liquid and provide the mechanical means to remove particlesfrom the wafer surface. At the same time, chemicals in the liquidprovide a slight surface etching and provide the right surfacetermination, such that once particles are dislodged from the surface bythe combination of etch and mechanical action of the megasonics on theparticles, these particles are not redeposited on the surface. Inaddition, chemicals are chosen such that an electrostatic repulsionexists between the surface termination of the wafer and the particles.

Until now, most megasonic irradiation has been applied to a bath inwhich the wafers are immersed. When using a cleaning bath filled with aliquid to immerse the wafer in, it is necessary to immerse multiplewafers at the same time to be efficient. Single wafer cleaning ispossible in a bath, but then the chemicals have to be reused, because ofthe volume of a single wafer bath.

So far, mechanical agitation in a single wafer cleaning method has beenachieved in several ways. At first, when wafers are completely flat,brushes can be used to scrub the wafer surface. However, this method isnot possible when the wafers have any topography (patterns) that can bedamaged by the brushes. Moreover, the brushes don't reach in between thewafer patterns. Megasonic energy, which is the preferred mechanicalagitation when patterns are present, can be applied to a liquid in anozzle and this liquid can then be sprayed on the wafer. When spraymethods are used in this way, the sonic pressure waves are confined tothe droplets of the spray where they then lose a lot of their power.When the droplets hit the wafer surface, most of the remaining sonicenergy is lost. Another method used is to apply megasonic pressure waveswith a quartz rod suspended over the wafer surface with the cleaningsolution building up between the rod and the wafer surface.

None of these attempts to apply megasonics to a single wafer surface issufficiently efficient as they do not reduce the single wafer cleaningtime enough, which is of the utmost importance. A single wafer cleaningapproach should be much faster than a batch cleaning process in order tobe competitive. Moreover, none of the current single wafer techniquesare able to clean sufficiently both the front and the backside of thewafer at the same time. The only known technique to clean the front andbackside at the same time is to immerse a batch of wafers in a bath andapply the acoustic waves from the sides of the wafers. In this manner,the acoustic waves travel parallel to the wafer surfaces to be cleaned.In silicon wafer cleaning, it is important to clean both sides of thewafer even though only the device side (front side) contains activedevices. Contamination left on the device side can cause amalfunctioning device. Contamination left on the non-device side(backside) can cause a number of problems. Backside contamination cancause the photolithography step on the front side to be out of focus.Contamination on the backside can cause contamination of the processingtools, which in turn can be transferred to the front side of the wafer.Finally, metallic contamination on the backside, when deposited before ahigh temperature operation, can diffuse through the silicon wafer andend up on the device side of the wafer causing a malfunctioning of thedevice.

Polysilicon or amorphous silicon is deposited on a silicon wafer fordifferent purposes. It can be the gate material of the transistor, or itcan be used for local interconnects or it can be used as one of thecapacitor plates in a capacitor structure. Most commonly, polysilicon oramorphous silicon is deposited on an insulating material, such assilicon dioxide. Polysilicon or amorphous silicon is usually depositedby a CVD (chemical vapor deposition) technique. The deposition ofpolysilicon or amorphous silicon usually occurs unselectively, that is,the entire wafer is covered with a layer of polysilicon or amorphoussilicon. After such a blanket deposition, the wafers are covered withphotoresist, the photoresist is exposed with UV light according to acertain designed pattern, and developed. Then the polysilicon oramorphous silicon is etched in a plasma reactor. The exposure of thephotoresist determines the pattern in which the polysilicon or amorphoussilicon will be etched. Usually, the polysilicon is used to conductcurrent from one place to another place or to collect charge as in acapacitor. In both cases, the dimensions are scaled down with every newgeneration of technology.

Until recently, dimensions not smaller than 0.3 μm (micron) were beingused. However, technologies using poly-line dimensions smaller than 0.3μm, such as 0.14 μm and even down to 0.1 μm are now being used. Thesepoly-line dimensions and capacitor plate dimensions are so fragile aconstruction that they are prone to breakage. These constructs are sofragile that agitation may break them and cause a defective chip. Afteretching and photoresist removal, such as with an oxygen plasma (i.e. theashing of the photoresist), the silicon wafers are usually riddled withparticles. These particles have to be removed before going to the nextdevice fabrication operation.

These particles are usually removed in a cleaning tool such as a wetbench. The particles are removed by immersing the wafers into a cleaningliquid and agitating the cleaning liquid with megasonic sound waves.This has worked well with poly-lines of 0.3 μm and above, however, whenusing poly-lines with dimensions smaller than 0.3 μm, megasonic soundagitation cannot be used as the megasonic sound agitation damages thesefragile structures. Therefore, only chemicals can be used to cleanparticles when these fragile structures are exposed to the cleaningliquid. Although, even simple immersion into a cleaning liquid withoutagitation does remove some of the particles, it cannot remove all of theparticles or even enough of the particles. Nevertheless, no alternativehas existed and therefore, this is the only cleaning technique used onthese fine structures.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed for single wafer processing thatapplies a cleaning or rinse solution to one or both sides of a waferpositioned above a platter. The wafer can be positioned in a bracket,the bracket rotated, and the platter can apply megasonic energy in theform of one or more frequencies to a side of the wafer. The bracket canhold the wafer at three or more points where wafer position ismaintained by gravity. At least one frequency applied to a 300 mm wafercan be at 5.4 MHz. The wafer side facing the platter may be thenon-device side, and the platter can generate the megasonic energy atone or more frequencies with one or more acoustic wave transducerspositioned on the platter backside.

The frequencies selected may be un-reflected by the platter and thewafer such that a large percentage of the megasonic energy will reachthe wafer side not facing the platter. While a cleaning/rinse solutionis applied to the wafer non-device side, a second cleaning/rinsesolution may be applied to the wafer device side. The megasonic energymay be pulsed and/or applied at varying power.

According to the present invention, chemicals area applied requiring lowvolumes and no-reuse of the cleaning and rinse chemicals. Applyingchemicals between the platter, having a dished out center, and thewafer, to be held in position by natural forces and then spinning thewafer to remove the chemicals is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of one embodiment of a wafer cleaningchamber.

FIG. 1B is an illustration of an alternate embodiment of the wafercleaning chamber.

FIG. 2A is an illustration of one embodiment of a megasonic single wafercleaning chamber.

FIG. 2B is an illustration of one embodiment of the center-section ofthe platter and the wafer having a flow of chemicals therein.

FIG. 3 is an illustration of an embodiment of a venturi nozzle design.

FIG. 4A illustrates in a top view, one embodiment of the rotatable waferholding bracket (bracket).

FIG. 4B illustrates the bracket in a 3D perspective view.

FIG. 4C illustrates the effects of airflow above and below the wafer inthe bracket rotating over a platter.

FIG. 5A is an illustration of a cross-section of one embodiment of theplatter.

FIG. 5B is an illustration of a bottom view of one embodiment of theplatter assembly showing a single acoustic wave transducer attached tothe platter.

FIG. 5C is an illustration of one embodiment having acoustic wavetransducers positioned in a strip fashion on the platter.

FIG. 6A illustrates one embodiment where a half circle of the plattersurface is coated with a first acoustic wave transducer that vibrates inthe 925 kHz range and the remaining platter half is covered with asecond acoustic wave transducer vibrating in the 1.8 MHz range.

FIG. 6B illustrates an alternate embodiment of the platter having twogroups of acoustic wave transducers in diagonal quadrants.

FIG. 6C illustrates an alternate embodiment where the platter has twogroups of transducers positioned on the platter in linear strips thateach runs substantially the diameter of the platter surface.

FIG. 7 is an illustration of wafer removal for one embodiment of thecleaning chamber.

FIG. 8 is an illustration of one embodiment where a plurality ofmegasonic frequencies is applied to quartz rods.

FIG. 9 is an illustration of one embodiment where a plurality ofmegasonic spray nozzles is used to transfer acoustic energy.

FIG. 10 is an illustration of one embodiment of an apparatus for batchprocessing a plurality of wafers using two or more megasonicfrequencies.

FIG. 11 is an illustration of a cluster of four single wafer cleaningapparatus that are positioned about a robot arm assembly.

FIG. 12 is an illustration of a single wafer cleaning apparatus.

FIG. 13 is an illustration of an alternate embodiment of a top chamber.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

An apparatus and method of use to provide single wafer cleaning isdisclosed. A process chamber (chamber) can process either or both a topand a bottom side of a single wafer in chip processing. The chamber canoffer high wafer throughput along with good process control whileproviding low use of cleaning solutions.

In one embodiment, a single wafer is positioned in a wafer holdingbracket (bracket) above a platter. Chemicals such as cleaning and rinsesolutions are transferred through the platter from below to contact thebottom side of the wafer. Sufficient chemical flow is provided to fill agap between the wafer and the platter. Once the gap is filled, littleadditional chemicals may be required, with the solution within the gapmaintained in position by natural forces such as surface tension andcapillary forces.

In another embodiment, a first group of chemicals (first chemical) aretransferred to the bottom side of the wafer while chemicals from adifferent source (second chemical) are transferred to a top surface ofthe wafer. In either embodiment mentioned above, megasonic sound wavescan be emitted from the platter to transfer through the first chemicalsflowing from below and strike the wafer bottom surface. In yet anotherembodiment, which can include elements of the above embodiments,megasonic sound waves are placed within chemicals that are applied tothe topside of the wafer where the solutions may be in the form of aspray or a thin film.

The use of acoustic wave transducers generating frequencies in themegasonic range has recently become common in wafer cleaning. Thedifference between ultrasonic cleaning and megasonic cleaning lies inthe frequency that is used to generate the acoustic waves. Ultrasoniccleaning uses frequencies from approximately between 20-400 kHz andproduces random cavitation. Megasonic cleaning uses higher frequenciesbeginning at between 350-400 kHz and may use frequencies well into theMHz range. An important distinction between the two methods is that thehigher megasonic frequencies do not cause the violent cavitation effectsfound with ultrasonic frequencies. Megasonic significantly reduces oreliminates cavitation erosion and the likelihood of surface damage tothe wafer. In general, the higher the frequency, the lower the damage tothe wafer.

Megasonic cleaning produces more controlled cavitation. Cavitation, theformation and activity of bubbles, is believed to be an importantmechanism in the actual particle removal process because cavitation hassufficient energy to overcome particle adhesion forces and causeparticles to be removed. Controlled cavitation becomes acousticstreaming which can push the particles away so they do not reattach tothe wafer. Megasonic cleaning may be improved by varying and/or pulsingthe input power to the megasonic transducers, which can provide bettercontrol over cavitation than applying power continuously at a constantlevel. Megasonic cleaning may be improved through the use of a pluralityof frequencies to be simultaneously generated, or by changing one ormore frequencies during the clean and rinse the cycles, or a combinationthereof. Megasonic cleaning may also be improved through a selection ofthe frequency or frequencies used.

In semiconductor processing, there are a number of occasions requiringprocessing of the wafer backside (non-device side) without processingthe front side (device side), such as to remove backside particlesbefore exposing the wafer to UV light from a lithography tool. Particleson the backside can cause depth-of-focus problems. In other occasions,deposition tools deposit materials on the front side on the wafers toform a film, but inadvertently, some deposits end up on the backside ofthe wafer. In other tools, such as copper electroplating tools, coppercontamination can end up on the backside of the wafer. In all thesecases, the backside has to be cleaned of particles and/or dissolvedmetals or certain layers have to be stripped.

FIG. 1A is an illustration of one embodiment of a single wafer cleaningchamber 100. Disclosed is an apparatus and method of use for exposingthe bottom side of the wafer 106 to cleaning, rinsing and dryingchemicals 112 without exposing the topside of the wafer 106 to anychemicals. In one embodiment, the wafer non-device side 114 is facingdown to be exposed to chemicals 112, while the wafer device side 116 isfacing up and is not exposed to chemicals 112.

In one embodiment, to initiate a wafer process cycle, a rotatable waferholding bracket (bracket) 148 translates along an axis 145 a distanceupward. A robot arm (not shown) holding the wafer 106 enters theinterior of the chamber 160 through an access door 158 and the wafer 106is placed in the bracket 148. The bracket 148 is then lowered so as toalign the wafer 106 horizontally a distance from a circular platter 108.The wafer 106, resting in the bracket 148, is parallel to the platter108 and located a distance from the platter 108, i.e. the gap. Theplatter 108 is flat where it faces the wafer 106 and therefore, thedistance separating the platter 108 and the wafer 106 is uniform. Thegap between the wafer 106 and the platter 108 may be in the range ofapproximately 1-5 millimeters (mm) and preferably approximately 3 mm.

In one embodiment, the wafer 106 when positioned in the bracket 148 canrest on three or more vertical support posts (posts) 110 of the bracket148. The vertical support posts 110 can contain an elastomer pad (shownin FIG. 4A later) to contact the wafer 106 directly. The wafer 106 isrotated while chemicals 112 are dispensed from below to contact thewafer backside 114. A tube 128 connects to a through hole (feed port)142 in the platter 108. As a result of wafer 106 rotation (spin),chemicals 112 applied to the wafer backside 114 are restricted fromreaching devices 121 on the wafer front side 116. In addition, a nozzle117 may move in over the wafer 106 o be positioned within approximately5 mm of the wafer surface and in the outer half of the wafer radius. Thenozzle 117 can apply a stream of inert gas 113 such as N₂ to the waferdevice side 116 to further limit chemicals 112 applied to the waferbackside 114 from migrating onto the wafer front side 116. Gravity andthe downward flow of air 123 from a filter 111 such as a High EfficiencyParticulate Arresting (HEPA) filter or an Ultra Low Penetration Air(ULPA) filter can act to maintain the wafer 106 positioned on the posts110. Chemicals 112 placed between the wafer 106 and the platter 108 canbe maintained in position by natural forces such as capillary action andsurface tension. As a result, a chemical flow rate required to maintainthe chemicals 112 against the wafer backside 114 can be reduced duringprocessing, which can allow for a small chemical use in each cycle andcan also allow for an efficient “no reuse” of chemicals 112. During thecleaning portion of the process, the wafer rotation may be stoppedallowing the wafer 106 to remain still while the cleaning chemicals 112contact the wafer bottom surface 114. The wafer 106 can be rotated,however, to wet out the wafer bottom surface 114 initially with thecleaning chemicals as well as for the rinse and dry cycles.

FIG. 1B is an illustration of an alternate embodiment of a single wafercleaning chamber 101. In this embodiment, the platter 108′ has adished-out center area 119 on the platter side facing the wafer 106. Forprocessing, chemicals 112 can be placed in the dished-out area 119 andthe wafer 106 can be positioned within the dished-out area 119 such thatthe wafer backside 114 is contacting the chemicals 112. This dished-outarea 119 of the platter 108′ can function to contain the chemicals 112and further reduce the amount of chemicals 112 needed during a processcycle. The dished out center 119 can be deep enough to submerge thebottom surface 114 of the wafer 106 while the top surface 116 of thewafer 106 remains outside of the chemicals 112. In one embodiment,approximately one half of the total surface area of the wafer 106 issubmerged within the chemicals 112. A nozzle 117′ may be placed in thetop area of the chamber 160 to flow a gas such as nitrogen onto thewafer topside. The nozzle 117″ may have to move or pivot to avoidcontact with the wafer 106 during wafer placement and removal as well asfor the rinse and spin cycles. The gas flow from the nozzle 117′ alongwith centrifugal forces if the wafer is spinning, can shift thechemicals 112 toward the wafer edge 115, further limiting migration ofany chemicals 112 onto the wafer top surface 116.

FIG. 2A is an illustration of one embodiment of a megasonic single wafercleaning chamber. FIG. 2B is an illustration of one embodiment of thecenter section of the platter and the wafer having a flow of chemicalstherein. The megasonic single wafer cleaning chamber 200 can incorporatethe methods, features and benefits of the single wafer cleaning chambers100 and 101 illustrated in FIGS. 1A & 1B. Within the cleaning chamber200, megasonic energy is generated by one or more acoustic wavetransducers (transducers) 202 attached to the platter 208 and themegasonic energy can pass into the wafer 206 through chemicals 212 incontact with both the wafer 206 and the platter 208. As a result, thewafer 206 can be cleaned with a variety of combinations that includewafer rotation, megasonic energy, and chemical action, all undertemperature control. Between and after the cleaning and rinsing cycles,the single wafer cleaning chamber 200 can dry the wafer 206.

The platter 208 has a topside 217 and a bottom side 219, with the set oftransducers 202 attached to the bottom side 219. The platter topside 217can be facing the wafer 206. The platter 208 is fixed in thisembodiment, but alternate embodiments can have the platter 208 able totranslate along the bracket rotation axis 245 to open the gap duringwafer rinse or dry cycles. The robot arm (not shown) can place the wafer206 in the rotatable wafer holding bracket (bracket) 248 such that thewafer device side 216 is facing up and away from the platter 208. Whenplaced in the bracket 248, the wafer 206 can be centered over and heldsubstantially parallel to the platter 208 to create the gap. The gapdistance is approximately 3 mm but can fall within the range ofapproximately 1-5 mm. Positioned beneath the platter 208 can be anelectric motor 222 for rotating the bracket 248. A through hole 225 canexist in the electric motor through which is passed the wiring 246 fromthe platter 208 as well as a tube 228 that can transfer the chemicals212 to the feed port 242.

Referring still to FIG. 2A, the platter 208 can have an approximate0.190″ diameter through-hole 242 that acts as a feed port for thechemicals 212 dispensed from below. This feed port 242 can be located atthe center of the platter 208 or the feed port 242 can be placedoff-center by up to a few millimeters (not shown). Attached to each ofthe acoustic wave transducers 202 can be a copper spring 244. The spring244 could be of a variety of shapes to maintain electrical contact suchas a wire coiled shape (shown) or a flexed foil constructed from sheetmetal (not shown). Soldered to the spring 244 free ends are the wiringleads 246 to form the electrical connections. The platter 208 can beconnected to the cleaning chamber 200 so as to act as ground for theelectrical connections 244 and 246 to the acoustic wave transducers 202.

In one embodiment, located above the platter 208 and the wafer 206, maybe positioned a nozzle 251. Through the nozzle 251 can pass a second setof chemicals 223, 224, 225, and 227 (second chemicals) duringprocessing. The nozzle 251 can direct a fluid flow 250 onto the waferdevice side 216 with each of the chemicals 223, 224, 225, and 227 in thecleaning process. The nozzle 251 can apply the chemicals 223, 224, 225,and 227 to the wafer 206 while the wafer 206 is not moving or while thewafer 206 is spinning. The nozzle 251 can apply the chemicals 223, 224,225, and 227 at a flow rate to maintain a coating of the chemicals 223,224, 225, and 227 on the wafer device side 216 surface with minimalexcess.

The nozzle 251 can apply a continuous chemical flow to maintain a filmthickness on the wafer 206 of at least 100 microns. To keep the chemicalfilm at the 100 microns thickness, the chemicals 223, 224, 225, and 227may be converted at the nozzle 251 into a mist having a particular meandiameter droplet size. All nozzle designs are limited as to how small adroplet size they can create. To meet the requirements of minimal fluidusage, a further reduction in droplet size may be required. One methodof reducing the droplet size beyond a theoretical limit is to entrain agas into the chemicals. The nozzle 251 can entrain or dissolve enough H₂gas 205 or any other gas from the group of O₂, N₂, Ar, or He into thechemicals 223, 224, 225, and 227 to further reduce the mean dropletsize. And in addition, entraining the gas 205 can have the added benefitof optimizing cavitation within the chemicals 223, 224, 225, and 227when the megasonics are applied.

FIG. 3 is an illustration of an alternate embodiment of a venturi nozzledesign. The nozzle, in the shape of a “showerhead”, is provided as anillustration of the use of a venturi to draw gases into the flow ofcleaning chemicals. The venturi shape can inject a gas source 305 suchas H₂ into the fluid steam 352 before the fluid stream 352 passes outholes 360 in a plate 358 in the nozzle 351 as a spray 350. Using thisapproach, the chemicals flow past a throat 354, which increases the flowrate thereby reducing the fluid pressure. A small hole (injector port)356 is placed in the throat 354 and is attached to a gas source 305 suchas H₂. As the fluid stream 352 passes by the injector port 356, the gas305 is drawn into the lower pressure of the fluid stream 352.Alternatively, the gas 305 may simply be injected into the fluid stream352 under sufficient pressure thereby avoiding the need for a venturidesign (not shown). Other approaches (not shown) for entraining gas intothe chemicals can be to bubble the gas into each cleaning fluid or tomist the cleaning fluids through a volume or stream of gas. Thegas-entrained chemicals then exit the nozzle 351 through a perforatedsurfaced 358 where the perforations 360 are sized to generate aparticular mean droplet diameter.

FIG. 4A illustrates in a top view, one embodiment of the rotatable waferholding bracket (bracket). FIG. 4B illustrates the bracket in a 3Dperspective view. The wafer 406 (shown in dashed line) can be held inplace by the bracket 448 to position the wafer 406 parallel to and nearthe platter (not shown for clarity). Initially, the bracket 448 can holdthe wafer 406 by gravity at four points 409 and 409′ along the waferedge 415 such that the wafer front side 416 and the wafer backside 414are clear of the bracket 448 structure and fully exposed to bothcleaning/rinsing liquids and thus to megasonic energy. The number ofpoints of contact 409 and 409′ for the bracket 448 with the wafer 406can be three or more and can be made with an elastomeric material suchas a plastic or rubber to friction grip the wafer 406 during the startand stop phases of rotation. In one embodiment, the contact points areO-rings that are positioned at the ends of bracket support posts (posts)411 where the posts 411 have been given an airfoil shape to minimizevibrations during high-speed rotations.

FIG. 4C illustrates the effects of airflow above and below the wafer 406in the bracket 448 rotating over the platter 408. When there are nochemicals between the wafer 406 and the platter 408 (portions of therinse cycle and the dry cycle), air can flow in circular swirls orpatterns 460 and 462 during wafer 406 rotation. The gap (not to scale)between the platter 408 and the wafer 406 limits the area of airflow andas a result, air flow circulating above the wafer 460 is at a differentflow rate than air flowing between the platter and the wafer 462. At thehigher rinse and dry wafer rotation speeds, the difference in flow rateprovides different pressures above and below the wafer 406 (Bernoulliforces), which can operate to provide a downward force acting on thewafer 406 that maintains the wafer 406 onto the bracket 448.

Referring again to FIGS. 2A and 2B, one embodiment of a method of userotates the bracket 248 and the wafer 206 while the first cleaningsolution 212 is applied from below to be in simultaneous contact withthe platter 208 and the non-device side of the wafer 214. The secondcleaning solution 223, 224, 225, and 227 is wetted out onto the deviceside 216 of the wafer 206. The acoustic wave transducers 202 generatemegasonic waves through the platter 208 into the first cleaning solution212, captured by the wafer 206 and the platter 208. The megasonic wavesmay be incident to the wafer non-device side 214 at an anglesubstantially normal (perpendicular) to the wafer surface 214. Apercentage of the megasonic waves, depending on the frequency orfrequencies used can pass through the wafer 206 to exit the wafer deviceside 216 and enter the second cleaning solution 223, 224, 225, and 227that is a film on the wafer device side 216. The megasonic waves actingwithin the second cleaning solution 223, 224, 225, and 227 can producecleaning on the wafer device side 216. For optimal throughput speed, thetotal area of the acoustic wave transducers 202 can be sufficient toprovide approximately between 80-100% area coverage of the plattersurface 219. The platter 208 diameter may be approximately the same sizeor larger than the wafer 206 diameter. The invention is scalable tooperate on a wafer 206 that is 200 mm (diameter), 300 mm (diameter), orlarger in size. If the wafer diameter is larger than the platterdiameter, the vibrations from the megasonic energy striking the wafer206 can still travel to the wafer 206 outer diameter (OD) providing fullcoverage for the cleaning action.

During the cleaning, rinse and dry cycles, the wafer 206 is rotated at aselected revolution per minute (rpm) about an axis 245 that runs throughthe bracket 248 pivot point. Additionally, to optimize any particularcycle, the wafer spin rate may be stopped or varied and the sonic energyvaried by changing any combination of the power setting, the frequencyor frequencies, and by pulsing. In one embodiment, the bracket 248,powered by the motor 222, can rotate the wafer 206 during cleaningoperations at an rpm of approximately between 10-1000 and during the dryand rinse cycles at an rpm of greater than 250 rpm where a range ofapproximately between 250-6000 rpm is preferable. Therefore, when thebracket 248 is in operation, the wafer 206 is seeing a first cleaningsolution 212 on the non-device side 214, a second cleaning solution 224on the device side 216, while the wafer 206 is being rotated andradiated with megasonic energy.

Continuing with FIG. 2A, acoustic waves can first strike the wafernon-device side 214 where no devices 221 exist that could be damaged bythe full force of the acoustic energy. Depending on the frequency orfrequencies used, the megasonic energy may be dampened to a degree whenpassing through the platter 208 and wafer 206 to exit into the cleaningor rinse chemicals 223, 224, 225, and 227 at the wafer device side 216.As a result, the megasonic energy striking the wafer non-device side 214may be powerful enough that only de-ionized (DI) water is used as thefirst cleaning solution 212.

A thin film (not shown) of the second cleaning solution 223, 224, 225,and 227 may be applied to wet the wafer device side 216 surface. If notDI water 225, the second cleaning solution 224 may be a strongerchemistry such as used in an RCA (Radio Corporation of America) cleaningprocess. The action of the megasonic energy on the device structures 221is confined to a small volume (thin film) that contacts the devicestructures 221, absorbs the sonic waves, and maintains usefulcavitation.

In an embodiment, megasonic energy is applied to the rotating wafer 206throughout the cleaning process. The megasonic energy is in a frequencyrange of 400 kHz-8 Mz but may be higher. The RCA type cleaning process,along with the prior use of an etchant such as hydrofluoric acid (HF)223 having a concentration of 0.5% by weight of HF, may be used on thewafer device side 216. The RCA cleaning process is commonly used and iswell known to those skilled in the art. The RCA process or a similarcleaning process may include a first standard clean (SC-1) cycle(NH₄OH+H₂O₂) 224, a rinse (DI water 225 ending with IPA vapor in N₂), anSC-2 clean (HCl+H₂O₂) 224, a rinse (DI water 225 ending with IPA vaporin N₂), and a dry cycle (blowing N₂ on the rotating wafer 206). Theapplication of IPA vapor in N₂ can be accomplished while DI water stillexists on the wafer. As a result, some of the previous cleaningchemicals still remain on the wafer, immersed in the DI water. The useof start of IPA vapor in N₂ blowing on the wafer can reduce the rinsetime since it begins prior to complete rinse, i.e. complete removal ofthe cleaning chemicals by the DI water. The effect of the IPA vapor inN₂ is to assist the rinse cycle and shorten the rinse cycle duration.The IPA vapor in N₂ 256 can be applied through a second nozzle 253 tosupport a rinse cycle on the top side 216 of the wafer. The secondnozzle 253 can be placed off-center to the wafer axis of rotation 245.In yet another embodiment (not shown), more than two nozzles can be usedwhich can be positioned in a variety of other patterns, such as equallydistant from the axis 245, so as to provide chemical and gas coverageonto the topside 216 of the wafer.

The wafer non-device side 214 may have the same cycles of clean, rinse,and dry or could use only DI water 212 in the clean and rinse cycles.The temperature of the cleaning chemicals, as well as the rinsingchemicals, etchants, and gasses can be between 15-85° C. during use. Adrain 262 may be provided within the cleaning chamber housing 260 tocollect the cleaning fluids. A cleaning chamber floor 263 may be angledtoward the drain 262 to improve flow of the chemicals 212, 223, 224,225, and 227 to the drain 262.

Cleaning of the wafer backside (non-device side) surface 214 may beaccomplished in a different manner. Because the acoustic energy ishigher on the backside of the wafer, no RCA type cleaning solutions 224may be necessary. The vibrations alone in water may be sufficient toseparate the particles from the wafer 206 and move them away. DI water212 may be selected as the medium to transfer the acoustic energy in thearea around the wafer backside 214 for both the cleaning and rinsecycles. In one embodiment, non-gas entrained DI water or even de-gassedDI water is preferred for use on the wafer backside 214. The DI water212 is fed through the tube 228, the feed port 242, and onto the waferbackside surface 214 at a sufficient rate to continually fill the areabetween the platter 208 and the wafer 206 which will guarantee constantfluid contact with the wafer surface 214. The DI water 212 can be vacuumdegassed before directing it to the fluid inlet port 242, by passing theDI water 212 through a membrane degassifier (not shown) such as withLiqui-Cel membrane contactors such as supplied by Celgard (Charlotte,N.C.). Alternatively, if a vacuum is placed on the gas side of themembrane, most of the dissolved gases can be removed from the incomingDI water 212. Alternatives to the rinse and dry cycles can include therinse cycle using IPA along with or instead of H₂O, and the dry cyclemay use wafer spinning and an inert gas such as N₂.

In one embodiment there is little use and no reuse of cleaningsolutions. This is a result of the small volumes of chemicals used inthe process such that it is efficient to use the chemicals once and thendiscard them. With such a small volume of chemicals used, the singlepass concept is economical and does not increase the burden to theenvironment. With the present invention, spraying a thin film may use1/10 or less the water volume as compared to existing wafer megasonicbatch processes using immersion. To reduce chemical use, the bracket 248may be rotated initially at a first speed to dispense the first chemical212 onto the non-device side 216 of the wafer 206 and to dispense thesecond chemical 223, 224, 225, and 227 onto the device side 216 of thewafer 206. Once dispensed, the bracket rotation speed can be slower thanthe first speed while megasonics are applied to the wafer non-deviceside 214. The bracket 248 can then be rotated at a speed higher than thefirst speed to rinse the wafer 206 and the bracket 248 rotated at aspeed higher than the first speed to dry the wafer 206.

After the chemicals are dispensed, the wafer rotation is slowed so thatthe first chemicals 212 can remain trapped between the wafer and theplatter as well as keeping the second chemicals 223, 224, 225, and 227wetted out on the wafer opposite side. In one embodiment, the initialwafer spin rate can in the range of approximately 50-300, where an rpmof 150 is preferable, while the cleaning solutions 212, 224, and 225 areapplied. In one embodiment, once the device side 216 of the wafer 206 iswetted with the chemicals 224 or 225, the wafer rotation speed may bereduced to a range of approximately 10-50, where an rpm of approximately15 is preferable, and/or the cleaning solutions 224 or 225 applied at alower rate, which in either case can reduce the cycle time and result inconserving chemical use. Finally, in one embodiment, after the cleaningprocess, during a rinse and/or dry cycle, the rpm can be increased toover 1000 to remove the chemicals remaining on the wafer 206.

The use of chemicals can be further decreased by wetting the wafersurface 216 with a finer spray of chemicals as opposed to a more coarsespray or even a solid stream of liquid. The finer spray can be achievedthrough an effective design of one or more nozzles 251 to apply thecleaning solution, by adjusting the temperature of the cleaning solutionapplied, by adjusting the chamber pressure acting on the spray, thefluid pressure in the nozzle 251, the chemical makeup of the cleaningsolutions 223, 224, 225 or 227, and the amount and type of entrainedgases 205 within the cleaning solution 223, 224, 225, and 227.

When the chemicals are not reused, the use of the platter 208 has thebenefit of containing the various liquids 223, 225, 224, and 227 thatwould otherwise fall by gravity from the wafer non-device side surface214. Containing the cleaning liquids 223, 224, 225, and 227 against thewafer 106 can reduce cleaning liquid use, optimize the acoustic energytransmitted from the platter 208 to the wafer 206 and can allow thecleaning liquids 223, 224, 225, and 227 to act longer on the wafersurface 214. Finally, cleaning solutions 223, 224, 225, and 227 appliedto the wafer non-device side, can be more dilute, i.e. made of a higherconcentration of water, which will further reduce cleaning chemicalconsumption.

After the last rinse cycle is complete there can be a dry cycle to drythe wafer. During the dry cycle, a few milliliters of isopropyl alcohol(IPA) vapor, mixed with nitrogen gas (N₂), can be injected through thefluid feed port 242 to contact the wafer device side 216 and non-deviceside 214. The IPA, having a lower surface tension than water, will wetout the surface better and form a smaller boundary layer. Thecombination of high wafer rpm, IPA vapor as a wetting agent, and N₂ gaspressure striking the wafer 206 reduces the drying time for the wafer206.

FIG. 5A is an illustration of a cross-section of one embodiment of theplatter 500. The platter 508 can be made of aluminum that is polishedand may have a surface finish of 16√ or smoother and having anapproximate 300 mm diameter. Alternatively, it should be noted that theplatter 508 can be made from a variety of materials such as sapphire,stainless steel, tantalum, or titanium. The platter 508 is approximately3.43 mm thick (530) and the platter front side 517 can be coated with aprotective fluoropolymer 534 such as Halar® (Ausimont USA, Thorofare,N.J.), having a coating thickness (536) of between 0.015-0.045″. Theplatter backside 514 can have one or more acoustic wave transducers 502bonded directly to the aluminum with an electrically conductive epoxyadhesive or a solder having an adhesive/solder thickness 540 ofapproximately 0.001-0.010″. The opposite side of each of the one or moreacoustic wave transducer 502 can be flexibly attached 544 to electricalwiring 520 to provide power at a frequency while the platter 508 can beconnected to ground.

FIG. 5B is an illustration of a bottom view of one embodiment of theplatter 500 showing a single acoustic wave transducer attached to theplatter. The shape shown is circular; however, any number of individualacoustic wave transducers 502, made into any shape such as square,round, or rectangular, can be used to meet area coverage andmanufacturing requirements. If more than one acoustic wave transducer502 is used, the acoustic wave transducers 502 can be positioned closetogether so as to provide the 80% or greater coverage of the platterbackside 514 surface area. The wafer 506 (dashed), upon receivingmegasonic energy to a portion of the wafer backside surface 507, cantransmit that megasonic energy to the entire wafer backside surface 507.This complete coverage of the wafer backside surface 507 can occur ifthe megasonic energy from the platter 508 is incident to between 50-100%of the wafer surface backside surface 507, however, optimal throughputcan require the 80-100% coverage, with 90-100% coverage preferred. Inone embodiment, 80% or greater acoustic wave transducer coverage on theplatter 508 is provided and as a result, megasonic energy will beapplied to the entire wafer backside surface 507 dramatically reducingthe cycle time and hence increasing the throughput of wafers. In anotherembodiment (not shown) the bracket can translate the wafer in lineartravel, without rotation, to pick up acoustic energy over the entirewafer surface.

Acoustic wave transducer thickness t (FIG. 5A) can be sized to generatesound at a particular frequency. When a signal, generated at thefrequency for which the transducer has been designed to respond, arrivesat the transducer, the transducer will vibrate at that frequency. Atypical acoustic wave transducer is made from a piezoelectric materialhaving a thickness of 0.098″, which is designed to respond to afrequency of 920 kHz. For a 300 mm wafer 506 (dashed to show a positionon the opposite side of the platter 508 in FIG. 5B), the frequency of5.4 MHz has a special utility in that the 300 mm wafer 506 istransparent for those sound waves. At 5.4 MHz±30%, the sound waves cantravel substantially through the wafer 506 to exit the opposite wafersurface. To obtain a frequency of 5.4 MHz, the thickness of the acousticwave transducer 502, as well as each thickness of all the other layers(platter 508 and adhesive/solder 540, FIG. 5A), are multiplied by afactor 920/5400=0.17 or alternatively the layer thicknesses of theacoustic wave transducer piezoelectric material, adhesive, and aluminumplatter are to be divided by a factor of 5.87. This will provide for atransducer to respond to a frequency of 5.4 MHz and for a reduced bounceback from the other layers of materials 508 and 540, that the sound mustpass through on its way to the wafer 506. An exception may be thethickness 536 of the fluoropolymer coating 534 (not to scale) which canbe kept similar in all embodiments. In one embodiment, the piezoelectricmaterial is a ceramic of lead zirconate titanate with the transducer 502manufactured by Channel Industries, Inc of Santa Barbara, Calif. In oneembodiment, an efficiency of at least 30% of the energy applied to thetransducers 502 can reach the wafer 506.

FIG. 5C is an illustration of one embodiment having acoustic wavetransducers positioned in a strip fashion on the platter. The acousticwave transducers 502 and 503 linearly placed on the platter backside 514can run a distance on the platter surface 514. The acoustic wavetransducers 502 and 503 on the platter backside 514, could be positionedas a strip that runs at least substantially the diameter (referring hereto the outer diameter) of the platter 508 covering approximately 40% ofthe platter backside 514 area. The acoustic wave transducers 502 maytransmit at a frequency that is different from the other acoustic wavetransducers 503. In one embodiment the acoustic wave transducers 502 canform one strip while the acoustic wave transducers 503 form a secondparallel strip. In an alternate embodiment (not shown) the acoustic wavetransducers 502 and 503 can be uniformly mixed. In another embodiment(not shown), the acoustic wave transducers could be a strip that runssubstantially a radius (R), the distance from the platter inner diameterto the platter outer diameter. For this embodiment, the acoustic wavetransducers 502 and 503 could cover approximately 20% of the platterbackside 514 surface area. As a result of less than 80% acoustic wavetransducer coverage of the platter, the wafer throughput may be reducedif the power is not increased to compensate, but complete coverage ofeach wafer with megasonics can still be maintained.

The effectiveness of cleaning by sound, in particular removingparticles, can be related to frequency, and different sized particlescan be more effectively removed with different megasonic frequencies.Currently, a large percentage of the particles to be removed from awafer (not shown) exist in the 0.3 μm (micron) and 0.1 μm sizes. It hasbeen determined that in cleaning wafers, the megasonic removal ofparticles in the 0.3 μm size range is efficient in the 900 kHz rangewhile the megasonic removal of particles in the 0.1 μm range isefficient in the 1.8 MHz range. In one embodiment, to provide twodifferent frequencies to a wafer for megasonic cleaning, a single signalis sent to all of the transducers that contains a combination offrequencies superimposed. The different transducers that exist on theplatter will each only respond to the corresponding frequency they aresized for. In this manner, within the single signal, individualfrequencies can be added and subtracted or power varied, for eachfrequency throughout the wafer processing cycles.

FIGS. 6A, 6B, & 6C illustrate one embodiment of acoustic wavetransducers 650 and 652 that output more than one frequency. It has beendetermined that there is a relationship between the size of the particleto be removed and the effectiveness of the megasonic frequency to removethat particle. When cleaning a wafer, particle sizes to be removed areoften in the 0.3 micron (μm) and 0.1 micron sizes. Megasonic frequenciesin the 925 kHz range have been found to be effective at removingparticles having a diameter of approximately 0.3 μm, and megasonicfrequencies in the 1.8 MHz range have been found to be effective atremoving particles having a diameter of approximately 0.1 μm. Theacoustic wave transducers 650 and 652 are attached to the platter 608where some of the acoustic wave transducers 650 output a frequency thatis different from the remaining acoustic wave transducers 652. FIG. 6Aillustrates one embodiment where a half circle of the platter surface614 is coated with a first transducer 650 that vibrates in the 925 kHzrange and the remaining platter half is covered with a second transducer652 vibrating in the 1.8 MHz range. As the wafer (not shown) rotates,the entire wafer is radiated with both frequency ranges. Even thoughthese transducers 650 and 652 are not vibrating at the 5.4 MHz frequencyto be transparent, sufficient energy can still reach the wafer to beeffective in cleaning.

A variety of transducer placement arrangements are possible to transfermultiple frequency acoustic energy to the wafer. A few additionaltransducer arrangements are described below but the invention is notlimited to them. FIG. 6B illustrates an alternate embodiment of theplatter 608 having two groups of transducers 650 and 652 in diagonalquadrants. FIG. 6C illustrates an alternate embodiment where the platter608 has two groups of transducers 650 and 652 positioned on the platterin linear strips that each runs substantially the diameter 654 of theplatter surface 614. In an embodiment, each transducer group 650 and 652covers approximately 20% of the platter surface area 614. In theembodiments using the half circle transducer placement (FIG. 6A), thequadrant transducer placement (FIG. 6B), and the linear strip placement(FIG. 6C), rotation of the wafer (not shown) will allow both frequenciesto strike at least 80% of the wafer surface. As a result of less than80% acoustic wave transducer coverage, the through put may

If the transducers 650 and 652 are not generating at the 5.4 MHzfrequency, i.e. transparent for the conditions that drove the 5.4 MHzselection, the various thicknesses making up the transducers 650, and652, adhesives 540 (FIG. 5B), and platter 608 can still be sized tominimize acoustic reflection and improve efficiency of the sound wavesreaching the wafer. With an embodiment having a first group oftransducers vibrating at a frequency approximately twice that of thesecond group of transducers, a platter thickness 530 (FIG. 5A) selectedto minimize reflection for one transducer group 650 frequency will beequally efficient at reducing reflection for the other transducer group652 frequency. The use of two frequencies has been given in the aboveembodiments for purposes of example, however, it should be appreciatedthat any number of different frequencies could be provided and that thepercent of coverage from each transducer type producing each of thefrequencies could be varied. When a platter thickness has been selectedthat minimizes reflection from one frequency, all of the otherfrequencies that will be applied can also have minimized reflection ifthe ratio of each frequency used is an integer multiple of the lowestfrequency.

FIG. 7 is an illustration of wafer removal for one embodiment of thecleaning chamber 700. During wafer 706 removal, an alternate bracket748, and the nozzle 751 can translate along an axis 745, moving upwardapproximately 1″ to allow for wafer 706 engagement with the externalrobot arm (not shown). Next, a cleaning chamber door 758 moves toprovide access to the cleaning chamber housing 760. With this opening,the robot arm can enter the cleaning chamber housing 760, engage andremove the wafer 706, and replace it with the next wafer (not shown) tobe cleaned. In this manner, the wafer 706 can be installed, cleaned, andremoved without requiring the system 700 to move complex components ofthe cleaning apparatus such as the platter 708, the electric motor 722,the fluid tubing 728 and the electrical wiring 746.

FIG. 8 is an illustration of one embodiment where a plurality ofmegasonic frequencies are applied to quartz rods. In this embodiment, achemical 806 is applied to the wafer 814 through a nozzle 816. A firstquartz rod 802 and one or more additional rods 804 may be placed closeto the wafer 814 so as to collect the liquid 806 between the quartz rods802 and 804 and the wafer 814. The quartz rods 802 and 804 can eachtransfer a different frequency to the liquid couplant 806 fromtransducers attached at the ends of each rod (not shown). The quartzrods 802 and 804 may be placed with their axes 808 and 810 runningparallel to the rotating wafer 814 to transfer sound pressure waves tothe wafer top surface 812 which may be the wafer non-device side or thewafer device side.

FIG. 9 is an illustration of one embodiment where a plurality ofmegasonic spray nozzles 902 and 904 are used to transfer acousticenergy. Each nozzle 902 and 904 imparts sonic energy to a water spray908 and 909 that strikes a wafer 906 rotating in a platter 907. Theacoustic energy is placed in water droplets 908 and 909, as imparted bythe nozzles 902 and 904, and the megasonic energized water can besprayed onto the rotating wafer non-device side surface 910. The platter907 may have a dished out center 912 to contain cleaning chemicals 911and in which the wafer 906 may “float”. The cleaning chemicals 911 canbe pumped into an area between the wafer device side 913 and the platter907. With this embodiment, more than one megasonic spray nozzle 908 and909 may be used in which a different frequency is imparted to one nozzle902 than is imparted by the other nozzle 904. As a result of waferrotation, the wafer 906 will receive both megasonic frequencies duringthe process. Alternatively, one or more megasonic frequencies can alsoby emitted from the platter 907 such that both sides of the wafer arereceiving acoustic energy directly, i.e. not just the acoustic energytransmitted through the wafer to the opposite side.

FIG. 10 is an illustration of one embodiment of an apparatus for batchprocessing a plurality of wafers using two or more megasonicfrequencies. A number of transducers 1004 and 1008 are positioned on achamber 1001 of the cleaning apparatus 1000. Transducers of a first type1004 generate at a first frequency while transducers of a second type1008 generate at a second frequency. The transducers of the first type1004 are positioned on a first chamber surface 1002 while thetransducers of the second type 1008 are positioned on a second chambersurface 1006 that can be approximately perpendicular to the firstsurface 1002. In this manner, sound waves generated by transducers ofthe first type 1004 and the second type 1008 both travel parallel to astack of wafers 1010 (only the top wafer is visible). To minimize waveinterference in the process chamber 1000 from the two frequencies,neither of the transducer sets are positioned 180 degrees from the otherset. In addition, one or both of the two frequencies can be pulsed. Inalternate embodiments, the transducers may be at angles other thanperpendicular. In one embodiment, a number of transducers, transmittinga number of frequencies, can each be positioned at angles less than 90degrees, i.e. acute angles, to meet constraints of the megasonic cleanerhousing 1012 shape and the number of frequencies to be generated. In analternate embodiment (not shown), the transducers 1004 and 1006 can bepositioned so as to be mixed on any surface.

FIG. 11 is an illustration of a cluster 1100 of four single wafercleaning apparatus 1101 that are positioned about a robot arm assembly1102. Attached at a side of the machine 1100 are a number of wafercartridges 1104, each holding a plurality of wafers 1106 to be cleanedor that have been cleaned. The cleaning processes of the cleaningchambers 1101 proceed in a sequence timed to optimize the use ofavailable space and the robot arm assembly 1102. One possible sequencehas the robot arm assembly 1102 take an unclean wafer 1106 from a wafercartridge 1104, install the wafer into a cleaning chamber 1101, remove aclean wafer 1106 from another process chamber 1101 and place the cleanwafer 1106 into another wafer cartridge 1104. This movement from processchamber 1101 to wafer cartridge 1104 to process chamber 1101 and so onwill optimize wafer 1106 cleaning times, however other sequencevariations may be used to select an optimal wafer cleaning cycle time.

FIG. 12 is an illustration of a single wafer cleaning apparatus. Thewafer cleaning apparatus 1200 is a stack of machinery. The top of thestack can be a filter 1210 where air flows through the filter 1210 usinga fan or a turbine. The filter 1210 can be placed on a top chamber 1220that positions the filter 1210 a distance from the cleaning chamber 1230to reduce the likelihood of chemical spray reaching the filter 1210. Thecleaning chamber 1230 can house the wafer holding bracket (not shown)along with the other equipment needed to processes the wafer. Beneaththe cleaning chamber 1230 can be located various electronics 1240 usedto control the cleaning process and at the bottom can be placed thecleaning and rinsing chemicals 1250 that feed up to the cleaningchamber.

FIG. 13 is an illustration of an alternate embodiment of a top chamber.In one embodiment, the air-flow from the filter above (not shown) ispartially re-directed 1320. A portion of the air 1310 flows down ontothe wafer 1325 (platter removed for clarity), however the remainingportion 1320 flows down a by-pass chamber 1330 of the top chamber 1350.A series of holes 1340 are spaced annularly and in line with thespinning wafer 1325. Chemicals 1345 that are spun off the wafer 1325during processing are drawn into the annular holes 1340 to flow down theby-pass chamber 1330. In this manner, the overall flow through thecleaning chamber 1300 is more balanced and chemicals 1345 can becollected with less contamination. Such chemicals 1345, collected withless impurities, may be considered for reuse.

It is well known in the art that sonic energy may bounce back or reflectwhen changing (material) boundaries. Therefore, it is to be expectedthat a particular acoustic frequency generated by a transducer throughthe transducer adhesive, the platter body, and the platter fluoropolymercoating will have many opportunities to reflect back and interfere withlater transmitted sonic energies. One approach is to design the variousthickness of materials to minimize or even eliminate this reflection.Another approach is to allow bounceback, perhaps even up to an 80%reflection and then pulse the transmitted sonic energy at a rate suchthat the new outgoing sonic energy does not run into the reflected sonicenergy. As previously mentioned, pulsing the sonic energy has theadditional advantage of improving cavitation and therefore acousticstreaming.

A thickness of a 300 mm wafer is nominally 0.775 mm. The elimination orreduction in reflection can be done by choosing the thickness of thelayers to be a multiple of λ/2, where λ is the wavelength of themegasonic energy applied to the wafer. Alternatively, for pulsing, theinterference by reflection can be eliminated by reducing the length ofthe signal pulse to less than 2 L/c with c the velocity of the acousticsignal in the layer and L the thickness of the layer. The velocity of anacoustic wave in silicon is roughly 8430 meters/second (m/s). Thereforethe length of the pulse or burst should be less than (0.775 mm)²/(8430m/s)=0.18 μs. Since this burst is very short, it is a better practice tochoose a frequency so that λ/2=0.775 mm and pulsing is not necessary.Since λ=8430 m/s/f with f the frequency, this gives a frequency ofapproximately 5.4 MHz.

After experimenting with 300 mm wafers, it was confirmed that theoptimum resonance frequency for transmission through the wafer withminimum reflection is 5.4 MHz. Therefore, in one embodiment this 5.4 MHzfrequency is used to transmit megasonic waves to the non-device side ofthe wafer. These frequency waves transmit almost without any reflectionthrough the platter and the wafer to the wafer side not facing theplatter, i.e. transparent frequency. For a different wafer thicknessthan the present 300 mm wafer thickness of 0.775 mm, 5.4 MHz would notbe the correct frequency. To generate a transparent wave through thewafer (and the layers of preceding materials), a formula based on thefollowing factors; the λ/2 thickness of layers ratio and the speed ofsound in silicon, coupled with the wafer thickness, may be used. Thegeneral formula for calculating the frequency that will be transparent(i.e. not bounce back) is: 4215±30% m/d, where m is meters and d=thethickness of the wafer in meters. In another embodiment, however, the4215 m/d formula for calculating frequency for the transparent wave maybe used to apply the frequency to the device side of the wafer. In thismanner, for a given wafer thickness, a sonic frequency having a wafertransparent to the wave could be applied directly to the wafer deviceside and/or the wafer non-device side. If more than one frequency isused that is transparent to the stack of materials the sound waves mustpass through to arrive at the wafer surface, it could be desirable tomake as many of the frequencies multiples of the lowest frequency aspossible. This would allow for the transparency of such frequenciespassing through the stack of materials. If one of such frequencies wastransparent to the wafer, then additionally all would have suchadvantage. This approach for generating transparent frequencies could beused in other wafer cleaning apparatus such as apparatus that totallyimmerse more than one wafer or apparatus that use one or more quartzrods or apparatus that uses one or more nozzles to place sonic energy inthe spray.

Particulate removal without poly-line, i.e. poly-silicon or amorphoussilicon, damage to fine structures, i.e. having dimensions less than 0.3μm, can be greatly reduced or eliminated through the use of a cleaningsolution used in conjunction with megasonic energy that is appliednormal to and striking the wafer backside surface. Megasonic energy inthe frequency ranges of 900 kHz or higher can completely suppress damageto the fragile poly-lines even when high acoustic power is applied. 700kHz or greater frequencies may be applied to the wafer backside that canprovide a megasonic power density of between 0.01 W/cm² (Watt percentimeter squared) and 10 W/cm² and preferably between 0.1-5.0 W/cm².Effective megasonic frequencies may be in the range of 700 kHz-2.0 MHzbut frequencies are preferably higher than 900 MHz and most preferablyapproximately 1.5 MHz±30%.

In an embodiment, the cleaning solution used (with megasonic energy), toreduce or eliminate poly-line damage, may be de-ionized water or thecleaning solution may be a mixture from the SC-1 cleaning process(mentioned above) and applied at approximately 60° C. The SC-1 cleaningprocess includes the cleaning mixture of NH₄OH+H₂O₂ added to water, andfor this embodiment, the cleaning mixture could consist of anammonia-to-hydrogen peroxide-to-water mixing ratio of approximately1:2:80 by volume. The ammonia supplied could be an approximate 28%solution by volume with water and the hydrogen peroxide supplied in anapproximate 31% solution by volume with water.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

1. An apparatus for processing a wafer, comprising: a bracket forpositioning and rotating the wafer about an axis; a platter alignedbeneath and parallel to the bracket, with the platter having a throughhole; a fluid source connected to the through hole for flowing a firstchemical within a gap between the wafer and the platter; and a pluralityof acoustic wave transducers positioned on the platter that are capableof transmitting a plurality of frequencies.
 2. The apparatus of claim 1,further comprising one or more nozzles positioned over the top of thewafer.
 3. The apparatus of claim 2, wherein at least one of the one ormore nozzles is connected to a source of a gas.
 4. The apparatus ofclaim 2, wherein at least one of the one or more nozzles is connected toa source of a second chemical.
 5. The apparatus of claim 1, wherein atleast one of the plurality of frequencies is a transparent frequency. 6.The apparatus of claim 2, wherein at least one nozzle is capable ofimparting acoustic energy to the second chemical flowing through thenozzle.
 7. The apparatus of claim 6, wherein the acoustic energy is at afrequency greater than 400 kHz.
 8. An apparatus for processing a wafer,comprising: a bracket for positioning and rotating the wafer about anaxis; a platter aligned beneath and parallel to the bracket, with theplatter having a through hole; a fluid source connected to the throughhole for flowing a first chemical within a gap between the wafer and theplatter; a plurality of acoustic wave transducers positioned on theplatter that are capable of transmitting a plurality of megasonicfrequencies; and at least one of the plurality of megasonic frequenciesis a transparent frequency.
 9. The apparatus of claim 8, wherein atleast one of the plurality of megasonic frequencies is a whole integermultiple of the lowest frequency.
 10. The apparatus of claim 8, whereinat least one of the plurality of megasonic frequencies is approximately5.4±30% MHz.